The present invention relates to a power supply with a high power factor suitable for a neon light, and more particularly to a high power factor power supply which outputs a non-discontinuous waveform.
Neon power supplies are generally designed to be as simple and inexpensive as possible. It is also desirable that they have a high power factor when operated from a normal AC source. As used herein, the term “high power factor” means a power factor of at least 0.9. Some designs use a rectifier-capacitor input filter to convert the incoming AC to DC to power the output high frequency converter, but such designs result in a low power factor, typically of the order of 0.6. The output waveform is, however, continuous (or to put it another way, non-discontinuous). In order to improve the power factor, some designs run from unfiltered full wave rectified AC. This gives a power factor in excess of 0.9, but the penalty is that the output waveform is discontinuous, as it goes to zero at the end of each half cycle. For a neon lamp load, the lamp must therefore re-strike every half cycle. In a normal situation this does not present a problem. However, there are disadvantages in not having a continuous output from the power supply, including (1) the re-striking of the neon lamp load generates more EMI than one operating continuously, and (2) with incompletely processed electrodes it is possible that the repetitive re-striking when using mercury-argon lamps can result in mercury being pumped away from the ends of the lamps. This then results in dimming of the end of the lamps. Typically this occurs when lamp makers use large diameter electrodes with small diameter glass, and the electrode conversion process cannot be adequately controlled.
One way of achieving a high power factor with a continuous output waveform is by utilizing a front-end boost converter, followed by an inverter to produce the output high frequency power. While this produces a high power factor and low harmonic distortion, the additional cost of the front-end converter makes the product less competitive in the market place.
There is a need for a neon power supply having a high power factor and which outputs a continuous waveform, without the additional cost of having a front-end boost converter.